1. Field of the Invention
This invention relates to steam engines as well as steam expanders that can be used as a part of a dual cycle engine for recovering waste heat, and especially to a method and apparatus for achieving higher efficiency in such engines and expanders.
2. Background of the Invention
Although steam engines can use a variety of liquid and lower cost solid fuels and inherently produce few toxic emissions, the thermal efficiency of current steam engines is significantly below that of internal combustion engines.
To achieve greater efficiency, our prior pending application Ser. No. 12/959,025, filed Dec. 2, 2010, provides a new Rankine operating cycle which can be referred to as a “zero clearance with zero compression” cycle or a Z-Z cycle in which the clearance can approach zero but provision must be made to keep the piston from hitting the head. Consequently, a near zero or quasi-zero clearance was used. While minimal compression is typically provided in both the present and the prior application, for various reasons some compression may be present during the last fraction of an inch before TDC due, for example, to the dimensions of the valves and parts used to operate valves.
U.S. Pat. No. 4,168,655 describes an engine with an automatic inlet valve in which the force used to close valve is overcome by knocking it ballistically off its seat, thus producing a high impact stresses in the valve and piston. In addition to eliminating high impact stress of U.S. Pat. No. 4,168,655, the piston of the present invention can be connected directly to a wrist pin without the need for external valving that adds substantially to the clearance volume. The prior patent also employs steam recompression which makes zero compression with zero clearance operation impossible, and routine variations in speed and pressure are able to interfere with repeatable cutoff control. It is also current practice as in U.S. Pat. Nos. 7,856,822 and 7,992,386 to admit steam through inlet passages or ports that extend out of the cylinder and therefore create a substantial clearance volume.
One major objective of the present invention is to find a way to provide a steam admission method and apparatus that concurrently provides inlet valve actuation with no clearance or almost no clearance between the piston and the end of the cylinder as well as eliminating connecting ducts or ports while simultaneously creating little, if any, compression in the cylinder between the piston and cylinder head at the end of the exhaust stroke such that the clearance is actually zero or approximates zero and, in this way, provide an engine constructed and operated such that the actual operating characteristics achieve maximum thermal efficiency in accordance with the zero clearance with zero compression operating cycle described herein and in our foregoing application Ser. No. 12/959,025 wherein indicated thermal efficiency which is better than that of the best prior operating cycle known, is given by the formula
      η    zz    =            Cycle      ⁢                          ⁢      Efficiency        =                            Work          NET                          m          ⁢                                          ⁢          Δ          ⁢                                          ⁢          h                    =                                    P            T                                              ρ              s                        ⁢            x            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            h                          ⁡                  [                                                                      x                  ⁢                                                                          ⁢                  n                                -                                  x                  n                                                            n                -                1                                      -                                          P                A                                            P                T                                              ]                    with terms defined as follows:ηZZ—Thermal efficiency of zero clearance zero compression cycleρS—density of supply steamPT—throttle pressurePA—condenser pressurex—cutoff=VO/VR Δh—enthalpy change in steam generatorn—polytropic exponent of expansion, e.g. 1.2m—steam mass at cutoffVO—cylinder volume at cutoffVR—cylinder volume at release